Alternatively, and just as reasonably, you could go with a Charm Leech Seed set. This set lacks the matchup consistency of the first one, but makes up for it in a slightly broader coverage for better general team consistency, as well as not having to rely on hitting Sleep Powder in most matchups. Two notable matchups this loses compared to the first are the Genesect matchup, which is a weak one on the team, and the Aegislash matchup, where you're going to have to rely on the mirror to win.
Code:
Venusaur-Mega @ Venusaurite
Ability: Thick Fat
EVs: 84 HP / 252 Def / 172 Spe
Timid Nature
IVs: 0 Atk
- Charm
- Leech Seed
- Sludge Bomb
- Synthesis
The high speed and timid nature put it above Gyarados and Dragonite provided they aren't running a Jolly nature, which is a pretty safe assumption anyway. After a charm this Venusaur can safely take a Supersonic Skystrike from both, and just kinda vibes with synthesis from there. The big risk running Sludge Bomb is you lose out on steel type matchups, but the alternative is losing to certain annoying grass types which this team does not enjoy either.